1. Technological Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, an intermediate transfer device and an image forming apparatus using the intermediate transfer device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities and typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum). An optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data. A developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image. The toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member in a process known as intermediate transfer. A cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium. Finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Generally, in known color-image forming apparatuses, four image bearing members (which may, for example, be photosensitive drums), one for each of the colors black, yellow, magenta, and cyan, are arranged in tandem facing a belt-type intermediate transfer member (hereinafter referred to as simply “intermediate transfer belt”), and multiple toner images of a respective single color are formed on the image bearing members. Then, the toner images are transferred onto the intermediate transfer belt so that they are superimposed one atop the other, thereby forming a composite toner image on the intermediate transfer belt. This process is known as a “primary transfer process.”
In an image forming apparatus using the intermediate transfer process, the composite toner image on the intermediate transfer belt is transferred onto a recording medium such as a sheet of paper at a secondary transfer nip at which the intermediate transfer belt contacts a secondary transfer member (which may, for example, be a secondary transfer roller) in a process known as a “secondary transfer process.” The intermediate transfer belt is formed into a loop and entrained around a plurality of rollers, one of which faces and presses against the secondary transfer member via the intermediate transfer belt, thereby forming the secondary transfer nip. This roller is known as a secondary transfer facing roller.
In such an image forming apparatus in which an image is formed on the recording medium through the first and the secondary transfer processes, transferability is decreased due to environmental changes. In view of the above, various approaches have been proposed in an attempt to solve the problem.
For example, in one approach, the secondary transfer roller is grounded, and a certain amount of electrical current having the same polarity as that of the toner is supplied to the secondary transfer facing roller disposed inside the loop formed by the intermediate transfer belt. In this configuration, even when an electrical resistance of the devices such as the intermediate transfer belt, the secondary transfer facing roller, and the secondary transfer roller fluctuates, degradation of the transferability in the secondary transfer process is prevented.
Although effective, there is a drawback to the above-described approach. That is, with recording media having different surface characteristics, it is difficult to optimally transfer the toner image onto the recording medium. When using a recording medium having a coarse surface such as the embossed sheet, a transfer potential of recessed portions of the recording medium is lower than that of projecting portions, so that the toner is not transferred adequately to the recessed portion, thereby yielding a resulting image with white spots. In order to enhance transferability when forming an image on a recording medium having a coarse surface such as an embossed sheet, the surface of the recording medium on which the toner image is transferred is charged to the opposite polarity to the polarity of the toner prior to the transfer process. In this configuration, upon transfer of the toner image, a transfer bias consisting of an alternating current (AC) voltage superimposed on a direct current (DC) voltage is supplied to the secondary transfer roller, and the rear surface of the intermediate transfer belt is grounded. Toner image can be transferred to the recessed portion in this configuration. However toner moves to the paper before the paper contacts the transfer belt because the paper is charged before the paper contacts the transfer belt. As a result a toner image on the paper is not clear.
JP7-114273A discloses a configuration using an alternating current in transfer. It discloses a transfer roller contacting a photoreceptor drum and separating roller. Moreover, the disclosed embodiment uses an alternating current of the same frequency and the same phase in the transfer roller and separating roller. However, implementing this embodiment is difficult because the current from transfer roller affects charging and developing by passing through a conductive base layer of the photoreceptor drum.
JP2005-181863A discloses a charge eliminator located near a paper sheet used to eliminate charge on the paper and to separate the paper from an intermediate transfer belt. The charge eliminator uses alternating current to eliminate the charge on the paper. The charge eliminator addresses troubles caused by separating the paper from the intermediate transfer belt. However the charge eliminator does not use alternating current in a secondary transfer roller and a secondary transfer facing roller. As a result, the white spots problem for emboss sheets still occurs. This reference does not use alternating current in the secondary transfer roller of secondary transfer facing roller.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good transferability regardless of fluctuation of the electrical resistance of devices as well as surface conditions of recording media. Moreover there is a need for preventing discharge between members having an alternating current applied thereto.